1. Field of the Invention
The present invention relates to a vacuum assisted system for loading a liquid product into a tank. More particularly, the present invention relates to vacuum assisted system for use on tanker loading racks in a petroleum product terminal.
2. Description of Prior Art
Vacuum assisted loading systems for use on loading racks and product terminals are provided to reduce vapor emissions. In these vacuum systems, the vapor expelled from a tank, truck or car during petroleum product loading are withdrawn through a vacuum hose and passed to a vapor recovery system which may either sorb the hydrocarbons in a sorbent bed or incinerate the hydrocarbons in a burner. The prime mover of the vapor from the tank to the vapor recovery system is the pressure of the product being loaded in the tank which may be in the order of thousands of gallons per minute. In prior emission recovery systems, the vapor recovery hose was hooked by a standard coupling to the tank. This coupling was not designed to be vapor or leak proof. Accordingly there was a significant amount of vapor escaping through the coupling. Some jurisdictions have a requirement that a vacuum be placed on the vapor recovery hose. This requirement may be that a minimum vacuum of -8.5" water be maintained on the vapor recovery hose while a tank is being loaded with a petroleum product. However, a maximum value is inherent and is set by the tank structure. Specifically, excessive vacuum may cause tank cells to collapse.
In one system, a large blower is installed in a header which ran overhead of a plurality of tanker loading bays, each bay having a downwardly extending conduit rig for connection to the tank of a tanker truck in a specific bay. This system senses the pressure in each downwardly extending loading system in each bay, and should the vacuum exceed what is needed in a specific bay the downwardly extending conduit system would be closed off from the overhead header. This system requires a relatively large and complex control scheme. Further, by taking a bay conduit system out of the vacuum scheme there is raised a possibility that there will be large variations in the header vacuum level. These variations will cause large fluctuations in liquid loading. One prior system includes an incinerator, and it is preferable to maintain a relatively constant hydrocarbon to air ratio to insure optimum combustion.